1. Field of Invention
The present invention relates to energy-saving windows, and more particularly to an apparatus for varying the light transmittance of window glazing.
2. Description of the State of the Art
Presently there exists approximately 19 billion square feet of windows in commercial and residential buildings, and another 600 million square feet of windows are being installed in new buildings throughout the United States each year, culminating in enormous cost and energy demands for air conditioning to negate unwanted solar heat gains through these windows. Static solar gain control coatings can be applied to windows; however, this practice is disadvantageous in that sunlight is blocked even when it is needed for lighting and heating during cold weather.
There have been some apparatus and methods developed for controlling the transmittance of light through window panes. For example, U.S. Pat. No. 4,768,865 to C. Greenberg et al., discloses a conventional electrochromic film on a window that is responsive to the application of an electric field to change from a high-transmittance, non-absorbing state to a lower-transmittance, absorbing or reflecting state. The Ito et al. patent, U.S. Pat. No. 4,832,468, discloses the use of an electrochromic coating for dimming automobile windows, including a plurality of solid electrochromic elements arranged in a horizontal abutting fashion and adhered to the glass window, each element being controllable independently of the others. C. Hashimoto et al., in his U.S. Pat. No. 4,958,917, discloses a specialized combination of two electrochromic cells which, together, are capable of reducing the transmittance of visible light to less than the usual lower limit of 7% for known electrochromic coatings.
While Greenberg, Ito, and Hashimoto teach the use of electrochromic coatings or controlling the transmittance of light through window panes, these coatings require external power supplies and, to be practical, the need for wiring into a building's electrical system. These coatings also absorb substantial mounts of the incident light and then radiate large amounts of the absorbed energy as heat to the interiors of the buildings in which they are installed, thus decreasing the effectiveness of the devices for blocking heat gain in the buildings and partially defeating the purpose for which they are used.
I. Mockovciak, in his U.S. Pat. No. 4,475,031, disclosed a self-contained sun-sensitive window made up of liquid nematic crystals (LC), sandwiched between two transparent sheets and powered directly by a solar cell. Liquid nematic crystals, however, are not effective in blocking heat radiation. Rather, they merely scatter light, thus making a window translucent, but not effectively blocking heat gain from the sun's rays. A further disadvantage of such liquid nematic crystal technology is that a constant source of energy is required to change the translucence of the window, thus requiring a substantial and continuous source of electric power as long as transparency instead of translucence is desired.